Several volunteered tracking codes and confirmation receipts as proof they complied.

The situation also highlights the byzantine manner in which Senate candidates — and only Senate candidates — file their campaign finance reports in the first place.

First, they print out paper copies their reports.

Next, they mail or hand deliver these paper copies to the Office of the Secretary of the Senate for processing.

Then, the Secretary of the Senate transfers the paper copies from Capitol Hill over to the FEC, which, in turn, ships them to a contractor for digital conversion.

When this conversion is complete, the FEC posts the data on its website, which can occur days or weeksafter a filing deadline. (Filings by presidential and House candidates, as well as political action committees, appear online instantaneously.)

The Senate's tortured data dance costs taxpayers about $500,000 annually, and the FEC's six commissioners — a typically fractious group — unanimously recommended to Congress in December that Senate candidates begin filing campaign disclosures digitally.

The FEC explained in a statement that it suspected 37 Senate committees had not filed their first-quarter campaign reports by the April 15 deadline. The FEC, which enforces campaign finance laws, learned from the Secretary of the Senate late Tuesday that 21 of those committees had indeed met the deadline — just as it was readying to send delinquent committees warning notices.

The FEC says those 21 notices were posted to its website but pulled from its email queue. At least some received them, however, as several committees that apparently shouldn't have received letters confirm they did.

Transparency advocates note that that Congress' upper chamber could easily avoid this kind of filing problem.

"If the Senate would enter the 21st century and file their campaign contribution reports electronically like House and presidential candidates do, we wouldn't have this problem," said Lisa Rosenberg, a lobbyist for the nonpartisan Sunlight Foundation, which advocates for greater government transparency.

Forty senators — mostly Democrats, but also six Republicans and two independents — have signed on to the latest incarnation of the Senate Campaign Disclosure Parity Act, which would require e-filing. But the Senate has yet to schedule it for a vote.

And while 21 senators voluntarily e-filed their campaign finance reports this month, the FEC doesn't consider those filings official.

They must submit paper copies to be considered in compliance with federal law.

Primary Source keeps you up-to-date on developments in the post-Citizens United world of money in politics. It features original, entrepreneurial reporting by Center staff, as well as examinations of primary source documents. Send tips to tips@publicintegrity.org.